I. Field of the Invention
This invention relates to a process for producing substantially unagglomerated single crystals of aluminum nitride.
II. Description of the Prior Art
There is considerable interest nowadays in materials suitable for the reinforcement of metal or ceramic matrix composites and similar materials. Composite materials of this type can be made to have very desirable properties, such as lightness of weight, high strength and resistance to high temperatures. Materials used for the reinforcement of the matrices must themselves have desirable properties, such as resistance to high temperatures and good resistance to attack by the matrix materials at high temperatures, as well as imparting a substantial reinforcing effect.
One material that is of interest as a matrix reinforcement is aluminum nitride. This material is especially suited for the reinforcement of aluminum matrix composites because of its high resistance to attack by molten aluminum. However, the reinforcing effect obtainable when using this material has been lower than expected because it has not been possible, at least on a commercial scale, to produce aluminum nitride in the form of particles ideally suited for reinforcement.
Aluminum nitride is used in the electronics industry, but the processes for the production of aluminum nitride for these applications generally result in the formation of sub-micron sized particles, which are too small for effective matrix reinforcement.
Most attempts to date to produce particles of aluminum nitride intended for reinforcement have resulted in polycrystalline particles which are then sintered and classified to obtain particles in the size range of about 10 microns. For example, individual particles can be produced by a carbothermal process for the production of aluminum nitride according to the following equation: EQU Al.sub.2 O.sub.3 +3C+N.sub.2 .fwdarw.2AlN+3CO
This reaction takes place at temperatures above 1700.degree. C. but, in order to grow crystals above 10 microns in diameter, the reaction temperature must be maintained above 1800.degree. C. However, when such temperatures are used, the product is unsatisfactory because a significant amount of carbon becomes trapped between agglomerating crystals of the AlN and the trapped carbon cannot be eliminated even by oxidation at elevated temperatures. Besides, a thick, almost sintered layer of AlN is formed on the surface of the particles, which hinders the diffusion of N.sub.2 inside the particle matrix as it is being formed, thus causing incomplete reaction in the interior of the particles.